This may not pertain to all 3x AAA flashlights, but with some calipers and common sense, you can probably check for yourself.
Step 1: The Background
I bought a cheap UV flashlight the other day to augment my pet detective toolset.
The initial problem was this: the flashlight is supposed to take 3xAAA batteries. Harummph, I say. Completely useless. I bought it anyway, figuring I'd modify it to take a useful battery type.
Step 2: Battery Carrier
This is the battery carrier. It holds 3 x AAA alkaline batteries in all their overpriced, anemic glory.
Whipping out the calipers, I found the battery carrier to be 52mm in length and 22mm in diameter. So the idea was to find a rechargeable lithium battery that would fit within those dimensions.
Step 3: Lithium Cells
Conveniently enough, lithium cells are labeled with their dimensions listed in millimeters.
One of the most common cells found in laptop batteries is the 18650 cell.
This means the cell is 18mm in diameter and 65.0mm in length.
Having a bunch of these, I tried to jam one in, but sadly it wouldn't fit. It was just too long.
So I went down to my local lithium battery super store, right down the street. Ermm, j/k. I went to the online electronic crap superstore based in Hong Kong. And I searched.
They had a 25500 lithium C cell, which was almost perfect. Well, just a tad too fat. I checked the internal diameter of the flashlight tube to be sure, and 22.5mm was the max it would accept.
After a few minutes it became apparent that the largest lithium battery that would fit is a 17340 cell. That would have left a horrible waste of space. I wasn't going to pay good money then wait 3 weeks for shipping time for that bad a solution.
Step 4: Making Space
The bottom cap of the flashlight contains a spring and a plastic spacer. I removed the spring by twisting it out while pulling on it. It came right out. I tried to put it back in for the picture, but it didn't want to cooperate. So be forewarned that this might be the point of no return.
Step 5: Dremel
I gutted the plastic to hold the battery. There is a little metal contact at the bottom, which I was careful not to maul.
Step 6: Done
When it's done, the laptop cell fits right into the hole, buying enough space to reassemble the flashlight.
Step 7: The Result
It's a perfect fit. The parts screw down all the way. The cell does not move when the flashlight is shaken.
Step 8: Afterthoughts
There was one small hitch. The plastic dust from the machining worked its way into the switch. I removed the rubber dust cover over the switch in order to shake it out. I destroyed the seal in the process. Oh well.
Step 9: Other Thoughts
I'm disappointed so far with the UV LEDs for my intended purpose. I think they emit too much visible light (as compared to a fluorescent blacklight) to be of much use in forensic work. So for now, the mystery of the unusual pet odor goes unsolved. But the investigation will continue.